Method of continuously casting steels

ABSTRACT

When molten steel jetted through an immersion nozzle into a mold for continuous casting is controlled by applying static field between opposed side walls of the mold for the continuous casting, this invention provides cast slabs having good surface and internal qualities by feeding molten steel to the mold for the continuous casting at a throughput of not less than 6 t/min and simultaneously applying a static field having a magnetic flux density of at least 0.5 T to a meniscus portion in the mold for the continuous casting and a static field having a magnetic flux density of not less than 0.5 T to a lower portion of molten steel jetted out from a discharge port of the immersion nozzle.

TECHNICAL FIELD

In the continuous casting of steel, molten steel received in a tundishis fed to a continuously casting mold through an immersion nozzle formedin a bottom of the tundish. In this case, the flow rate of molten steeljetted out from a discharge port of the immersion nozzle is considerablylarge as compared with the casting rate of steel, so that inclusions orbubbles in molten steel are apt to be deeply penetrated into a craterand hence it is not avoided to cause internal defects. Further, there isa problem of remelting solidification shell, while the jet of moltensteel in an upstream flowing direction (contrarotating flow or the like)among the jets of molten steel upheaves a meniscus portion in the moldto promote variation of molten steel surface to thereby entrap moldpowder thereinto, which has a remarkably bad influence upon the qualityof the resulting cast slab and the casting operation.

This invention stably provides cast slabs having improved surface andinternal qualities by mitigating variations in the molten steel surfacein a mold for continuous casting, entrapment of powder, entrapment ofinclusions and the like to improve the internal quality and soundingsurface properties when molten steel is cast at a higher throughputexceeding two times of the conventional throughput for molten steel anda higher speed.

BACKGROUND ART

In order to control the flow of molten steel jetted out from theimmersion nozzle, it was usual to contrive the shape of the dischargeport in the immersion nozzle, or to decrease the pouring rate of moltensteel.

However, it was difficult to completely prevent quality defectsresulting from inclusion or the like in molten steel by simply changingthe shape of the discharge port or decreasing the pouring rate of moltensteel.

As a prior art relating to this point, JP-A-57-17356 discloses a methodof applying a braking force to the flow of molten steel jetted out fromthe immersion nozzle by arranging a device for generating a static fieldin the mold for the continuous casting, and JP-A-2-284750 discloses atechnique of applying a braking force to the flow of molten steel jettedout from the immersion nozzle by Lorenz force produced through aninteraction between current and magnetic field induced by applying astatic field to the entire mold for the continuous casting.

In the technique disclosed in JP-A-57-17356, when the braking force isapplied to the jet of molten steel, the flowing direction is changed todisperse the energy inherent to the jet of molten steel as if the jet ofmolten steel collides with a wall and hence the uniform flow can not beobtained. Further, the jet of molten steel escapes in a direction havingno static field, so that the satisfactory result can not be obtained.

In the technique disclosed in JP-A-2-284750, it is possible to attainthe uniformization of molten steel jetted out from the immersion nozzleand the variation of molten steel surface on the meniscus portion can bemade small, so that the surface and internal qualities of the cast slabcan be improved to a certain extent, respectively. However, when thehigh-speed casting is carried out under a condition that the throughputof molten steel exceeds 2 times of the conventional throughput, therestill remaining the following problems.

1) When using a multihole type immersion nozzle, the occurrence ofdeflected flow in a mold accompanied with the flow of molten steeljetted out from the immersion nozzle can not be avoided.

2) In case of the multihole type immersion nozzle, when the clogging ofthe nozzle is caused with the increase of the flow rate of molten steeljetted, the deflected flow in the mold becomes large and hence thecontinuous casting can not stably be attained.

3) In case of the multihole type immersion nozzle, the contrarotatingflow at a short side of the mold becomes high speed accompanied with theincrease of the flow rate of molten steel jetted, so that the variationof molten steel surface becomes large and the entrapment of powder cannot be avoided. Moreover, the use of single-hole type immersion nozzlecan be considered. In the latter case, when the static field is appliedto a lower zone of molten steel jetted, the contrarotating upstream ofmolten steel is generated through an influence of reflection current(induction current flowing in a direction of promoting the jet of moltensteel) in the mold to cause the variation of molten steel surface andhence powder is entrapped.

4) Since the disorder of molten steel becomes large during theoscillation of the mold, the depth of oscillation mark becomes deeperand also the oscillation mark becomes disordered, so that surfacedefects (coil defect) are frequently created in the resulting rolledsteel sheet.

5) Since the molten steel surface is rippled inside the mold to disorderthe oscillation mark, it is difficult to uniformly supply powder andhence it is apt to cause restraint breakout due to the occurrence ofsticking or the like.

6) There is a fear of remelting solidification shell by the flow ofmolten steel jetted out from the immersion nozzle.

Recently, there is proposed a continuously casting method through theapplication of static field to a lower end portion of a mold for thecontinuous casting (JP-A-7-51801, JP-A-7-51802, JP-A-59-76647,JP-A-62-254955, Iron Steel Eng., May (1984), pp 41-47, JP-A-6-126399), acontinuous casting method using two nozzles while applying the staticfield to the lower end of the mold for the continuous casting(JP-A-5-277641) and the like.

These techniques are intended for not only the continuous casting ofordinary steel but also the casting of clad steel. In these techniques,it is possible to decrease the flow rate by applying the static field toan adequate zone (zone near to solidification shell at a side ofshort-side wall in the mold for the continuous casting or the like) forthe flow of molten steel jetted out from the immersion nozzle, so thatthese techniques may sufficiently be applied to the continuous castingof ordinary steel. In any case, the value of the static field is notmore than 0.5 T, so that it can not be adapted to the high-speed castingat a throughput of 6-10 t/min. Therefore, it is a disadvantage that thecastable quantity is very slight without generating defects in theproduct.

In order to increase magnetic flux density and mitigate power cost,JP-B-63-54470 discloses a technique of exchanging the conventionalnormal conducting electromagnet with a superconducting electromagnet.

However, when the conditions of applying the static field are badirrespectively of the normal conducting electromagnet or thesuperconducting electromagnet, there are rather frequently generateddefects. Particularly, when the high-speed casting is carried out bychanging the throughput of molten steel from about 5 t/min usually usedto more than 6 t/min, the restrictions in the operation become severerfrom problems such as disorder of molten steel surface, entrapment ofinclusions and the like. In this technique, there is no description onmagnetic field applying conditions and casting conditions required forobtaining cast slabs having no defect.

In this connection, a casting method using a superconductingelectromagnet and a cuspid magnetic field is disclosed in JP-A-3-94959.According to this method, the intensity of the magnetic field is about0.15 T at most and is fairly small as compared with the case of usingthe conventional electromagnet and also the application system of themagnetic field is cusp, so that it is impossible to control thevariation of molten steel surface in the mold for the continuous castingquestioned in the high-speed casting.

Moreover, a method of casting slabs having less defects by applying astatic field having a magnetic field intensity of 0.5 T at maximum to alower end of the mold is disclosed in JP-A-4-52057, whereby it ispossible to mitigate the entrapment of bubbles and inclusions ascompared with the conventional case. However, the casting conditions arethe same as in the conventional technique, so that it can not cope withthe high-speed casting.

Up to the present, there is no proposal for solving the above items1)-6) in order to realize the high-throughput, high-speed casting.

It is an object of the invention to solve the aforementioned problemswhen the high-speed casting is carried out at a high throughput and toprovide a novel method of continuously casting steel to produce carefreecast slabs suitable for DHCR process (direct hot charged rollingprocess) or CC-DR process (continuous casting rolling process) as wellas an apparatus suitable for carrying out this method.

DISCLOSURE OF INVENTION

This invention is a method of continuously casting steel by controllinga jet of molten steel fed through an immersion nozzle into a mold forcontinuous casting while applying a static field between opposed sidewalls of the mold for the continuous casting, characterized in thatmolten steel is fed into the mold for the continuous casting at athroughput of not less than 6 t/min, and that an air-coresuperconducting electromagnet is used to simultaneously apply a staticfield having a magnetic flux density of at least 0.5 T to a meniscusportion in the mold for the continuous casting and a static field havinga magnetic flux density of not less than 0.5 T to a lower portion ofmolten steel jetted out from a discharge port of the immersion nozzle.

In the invention, the static field is applied to a full region inwidthwise direction of the mold including the meniscus portion and thelower portion of molten steel jetted.

Further, the continuous casting is carried out by oscillating the moldfor the continuous casting so as to satisfy S·F≧450 (S: up and downstrokes (mm) of the mold for the continuous casting, F: oscillationnumber (cpm)) in the feeding of molten steel through the immersionnozzle.

A gas (gases such as Ar, N₂, NH₃, H₂, He, Ne and the like are used aloneor in admixture) is blown into the immersion nozzle according to acondition of 0.5Q≧f≧20+3Q (f: gas blowing amount (N1/min), Q: throughputof molten steel (t/min)).

As the immersion nozzle is used a single-hole type straight nozzle.

In the invention, when the air-core super-conducting electromagnet isused as an electromagnet applying the static field, support members areseparately arranged in the mold for the continuous casting and thesuperconducting electromagnet, and a distance between magnetic poles ofthe superconducting electromagnet is changed so as to approach with eachother or separate away from each other in accordance with castingconditions to adjust the magnetic flux density of the static field.

It is particularly advantageous that current is applied to the mold forthe continuous casting, and that an induction current generated by theapplication of the static field is taken out from a short-side wall ofthe mold for the continuous casting and supplied to the other short-sidewall thereof to circulate the induction current.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1a is a graph showing the relation between the temperature ofmolten steel surface in a mold for continuous casting and a magneticflux density (i.e. magnetic flux density when static field is applied toa lower portion of molten steel jetted).

FIG. 1b schematically shows a tundish and a mold and their accompanyinglabels T_(t) and T_(m) for the tundish temperature and molten steelsurface in the mold.

FIG. 2 is a graph showing a relation between a nozzle clogging and amagnetic flux density (i.e. magnetic flux density when static field isapplied to a lower portion of molten steel jetted).

FIGS. 2b and 2c schematically show discharge ports of a nozzle, withoutand with inclusions, respectively.

FIG. 3 is a graph showing a relation between an occurrence ratio of coildefect and a magnetic flux density (i.e. magnetic flux density whenstatic field is applied to a lower portion of molten steel jetted).

FIG. 4 is a graph showing a relation between an occurrence ratio ofbreakout and a magnetic flux density (i.e. magnetic flux density whenstatic field is applied to a lower portion of molten steel jetted).

FIG. 5 is a graph showing a relation between a nail depth in oscillationmark portion and a superheat of molten steel.

FIG. 5b shows a portion of a nail-like shell on the surface of a steelsheet.

FIGS. 6a and b are diagrammatic views illustrating the construction ofan equipment suitable for carrying out the invention.

FIGS. 7a and b are diagrammatic views illustrating the construction ofanother equipment suitable for carrying out the invention.

FIGS. 8a and b are diagrammatic views illustrating the construction ofthe other equipment suitable for carrying out the invention.

FIGS. 9a and b diagrammatic views illustrating the construction of stillfurther equipment suitable for carrying out the invention.

FIG. 10 is a diagrammatic view illustrating the construction of asuperconducting electromagnet for the generation of static field.

FIG. 11 is a diagrammatic view illustrating the construction of a moldfor continuous casting suitable for carrying out the invention.

FIG. 12 is a perspective view of FIG. 11.

FIG. 13 is a graph showing a relation between a distance betweenmagnetic poles and a relative magnetic flux density of static field.

FIG. 14 is a graph showing a relation between a magnetic flux density(index) and a deformation quantity (index) of a cooling plate in a mold.

FIGS. 15a and b are partial section views of a main part of acontinuously casting apparatus according to the invention, respectively.

FIG. 16. is a diagrammatic view illustrating a main part of anelectrode.

FIGS. 17a and b are diagrammatic views illustrating the construction ofa mold for continuous casting suitable for carrying out the invention.

FIGS. 18a and b are diagrammatic views illustrating the construction ofanother mold for continuous casting suitable for carrying out theinvention.

FIGS. 19a and b are diagrammatic views illustrating the construction ofthe other mold for continuous casting suitable for carrying out theinvention.

FIG. 20 is a graph showing a relation between a magnetic flux densityand a current.

FIG. 21 is a graph showing a relation between a magnetic flux densityand an occurrence ratio of cold rolled coil.

FIG. 22 is a diagrammatic view illustrating a continuously casting stateaccording to the conventional system.

FIGS. 23a, b and c are schematic views illustrating states ofaccelerating a jet of molten steel through reflection current,respectively.

FIG. 24 is a diagrammatic view illustrating a preferable construction ofa mold for continuous casting used in the invention.

FIG. 25 is a diagrammatic view illustrating another preferableconstruction of a mold for continuous casting used in the invention.

FIG. 26 is a schematic view showing a flow of induction current.

FIG. 27 is a diagrammatic view illustrating the construction of a moldfor continues casting provided with an air-core superconductingelectromagnet.

FIGS. 28a and b are diagrammatic views illustrating a main part of asuperconducting electromagnet, respectively.

FIG. 29 is a diagrammatic view illustrating the construction of anothermold for continues casting provided with an air-core superconductingelectromagnet.

FIG. 30 is a diagrammatic view illustrating a main part of asuperconducting electromagnet.

FIG. 31 is a graph showing a relation between a magnetic flux densityand an occurrence ratio of surface defect.

FIG. 32 is a graph showing results of measurements of inclusions in acast slab.

FIG. 33 is a graph showing results of measurements of occurrence ratioof breakout.

FIG. 34 is a graph showing results of measurements of surface propertiesof a cast slab.

BEST MODE FOR CARRYING OUT THE INVENTION

FIGS. 1a, 1b and FIGS. 2a, 2b and 2c show results examined on relationsof a temperature of molten steel surface in a mold (index) and a nozzleclogging in an immersion nozzle (index) to a magnetic flux density ofstatic field applied in the continuous casting (application type ofmagnetic field: full width type of two up-and-down stage L₁ =250 mm, L₂=250 mm, see FIG. 8, magnetic flux density: applicable of 0-10 T) whenthe continuous casting is carried out under conditions that an amount Qor throughput of molten steel fed through the immersion nozzle (C: 20-30ppm, Mn: 0.1-0.2 wt %, P: 0.01-0.012 wt %, S: 0.006-0.010 wt %, Ai:0.032-0.045 wt %, T.O: 22-32 ppm) is 4 t/min, 7 t/min, or 10 t/min, atemperature of molten steel in a tundish T_(t) : 1555°-1560° C., 1charge: 230 t, a size of a mold: 260 mm×1300 mm, a vertical bending typecontinuous casting machine (vertical portion: 3 m), an immersion nozzle:two-hole nozzle, a nozzle size: 70 mm in inner diameter, a size of adischarge port: square of 70 mm×80 mm, a nozzle angle: 15° downward, andpresence or absence of blowing a gas (Ar gas) for the prevention ofnozzle clogging, respectively. Moreover, the magnetic flux density inFIGS. 1a and 2a is adjusted to 0.5 T in a meniscus portion and a rangeof 0-5 T in a lower portion of molten steel jetted. As a gas blowingamount, stroke and oscillation condition, FIG. 1a is gas blowing amount:20±2 Nl/min, stroke of mold: 8-10 mm and oscillation: 187-257 cpm, andFIG. 2a is gas blowing amount: 22±4 Nl/min, stroke of mold: 7-9 mm andoscillation: 170-220 cpm.

When the jet of molten steel is controlled by applying a static field insuch a manner that the magnetic flux density is 0.5 T in the meniscusportion and 0.5 T in the lower portion of the molten steel jet, thelowering of the temperature of molten steel surface in the mold becomessmall (FIG. 1), and the nozzle clogging is reduced by rectifying actionof the molten steel jet at the discharge port of the nozzle (FIG. 2).

Particularly, the above tendency becomes remarkable in case of blowingthe gas. Even if the gas blowing is not carried out, the effect arisesat 0.5 T and becomes conspicuous near to 0.7 T. Near to 1.0 T, theeffect approaches to the case of blowing the gas, and hence the loweringof the molten steel surface temperature is small and the nozzle cloggingis substantially eliminated. Since the gas is blown into molten steel asbubbles, the floating effect is first developed by blowing at a flowrate of not less than 0.5Q Nl/min (Q: throughput). When a great amountof the gas is blown, the floating effect becomes large and the controlof the molten steel jet is easy, but as the amount of bubbles per unitvolume becomes too large, current produced in the magnetic field hardlypasses and the braking effect of the magnetic field drops down.Therefore, when the gas is blown into the immersion nozzle, thethroughput of molten steel as Q (t/min) is about 20+3Q as an upperlimit.

When the static field is applied so as to have a magnetic flux densityof about 0.5-1.0 T, it is particularly preferable that the gas is blownat 0.5Q≦f≦20+3Q (f: gas blowing amount (Nl/min)).

In the gas blowing, the lower limit is determined from the floating ofinclusions and the degree of requesting the temperature rise of moltensteel surface, while the upper limit is determined from a point ofpreventing the entrapment of inclusions transferred with the jet underthe application of the magnetic field through solidification shell or apoint of preventing the increase of inclusions due to disorder of moltensteel surface.

As a gas to be blown, Ar gas is acceptable usual, but a mixed gas of Arand N₂ may be used. In addition, various gases capable of producing thefloating effect through bubbles and giving the braking force to the jetof molten steel and causing no contamination of molten steel may beused, so that the kind of the gas is not particularly restricted.

As to the static field applied to control the jet of molten steel, it isimportant that the magnetic flux density is not simply increased but thelength of the magnetic field applied to the jet of molten steel ismaintained in a particular range.

The application length of magnetic field capable of controlling the jetof molten steel is considered to be a range capable of giving a brakingforce for stopping or decelerating kinetic energy of flowing moltensteel. In general, energy E of magnetic filed applied to the flowingconductive fluid can be represented by E ∝ (V₁ /ρ)B² ·L when an averageflow velocity of the fluid is V₁, a magnetic flux density is B, aresistivity of the conductive fluid is ρ and an application length ofmagnetic field is L (see FIGS. 6-8). In case of the high-speed castingat a throughput of molten steel of not less than 6 t/min, theapplication length L of the magnetic field required for decreasing theflow velocity of molten steel can particularly be represented as k·Q/B≦L(k:0.55, L(cm), B (T), Q (t/min)) by determining a constant ofproportionality from model experiments and the like.

In the invention, it is favorable that the minimum value of the lengthof magnetic filed applied to the meniscus portion is about 50 mm andalso the minimum value of the length of magnetic field applied to thelower portion of the molten steel jet is about 50 mm.

When the static field is applied by using an air-core superconductingelectromagnet, the application length L of magnetic field is a distancebetween upper and lower ends of winding in the electromagnet, and themagnetic flux density B is maximum at 1/2 of a thickness of a castingmold in the application length L of the magnetic field. Therefore, whena plurality of electromagnets for the application of magnetic field areused, L is L=L₁ +L₂. . . +L_(n).

When static field is applied to a mold for continuous casting in such amanner that the magnetic flux density in the meniscus portion is notless than 0.5 T and at the same time the magnetic flux density in thelower portion of the molten steel jet is not less than 0.5 T, thevariation of molten steel surface due to the contrarotating flow ofmolten steel in case of using the multihole type immersion nozzle issuppressed, and also the downstream of molten steel discharged anddownstreamed from the immersion nozzle is rectified, so that the flowingof molten steel in the nozzle and the discharge port thereof becomesuniform and hence a fear of nozzle clogging is less.

In the case of the single-hole type immersion nozzle, when static fieldof not less than 0.5 T is simultaneously applied to the meniscus portionand the lower portion of the molten steel jet, the variation of moltensteel surface due to the contrarotating upstream of molten steel issuppressed and also the collision of the molten steel jet withsolidification shell, which is concerned due to the high-throughput,high-speed casting, is avoided and hence a risk of remelting isconsiderably mitigated.

FIG. 3 and FIG. 4 show results of tests evaluating the occurrence ratioof coil defect and occurrence ratio of breakout to the magnetic fluxdensity (in FIG. 3, gas blowing amount: 18±2 Nl/min, stroke: 6-8 mm,oscillation number: 240-260 cpm; in FIG. 4, gas blowing amount: 28±2Nl/min, stroke: 6-8 mm, oscillation number: 240-260 cpm; the otherc6nditions are the same as in FIGS. 1 and 2). When a static field isapplied at a magnetic flux density of not less than 0.5 T to both themeniscus portion and the lower portion of molten steel jet, theentrapment of powder and the occurrence ratio of breakout become verysmall.

Moreover, when the magnetic flux density of a static field applied tothe meniscus portion is not more than 0.35, even if the throughput isnot less than 6 t/min, the occurrence ratio of coil defect is not lessthan 0.25% irrespectively of single-hole nozzle and multihole nozzle.

In FIG. 5a shows the relation between superheat of molten steel surfacein a mold for continuous casting and the nail depth, as shown in FIG.5b, of an oscillation mark in the surface of a cast slab when themagnetic flux density is 0-1.25 T. As seen from FIG. 1a and FIG. 5a, thenail depth is mitigated by simultaneously applying a static field havinga high magnetic flux density to both the meniscus portion and the lowerportion of molten steel jet to maintain the superheated superheat ofmolten steel surface at a high level. By mitigating the nail depth isdecreased amounts of inclusion, powder and bubbles caught with the nailportion, so that it is considered to lower the defect ratio in the coldrolled coil product.

In the invention for the high-speed casting at a throughput of moltensteel of not less than 6 t/min, the continuous casting is carried out soas to satisfy a condition of S·F≧450 (S: up-and-down stroke of a moldfor continuous casting (value between maximum value and minimum value ofamplitude)(mm), F: oscillation number (cpm)) during the feeding ofmolten steel through the immersion nozzle. Because, when conducting thehigh-speed continuous casting aiming at the invention, the stabilizationof the molten steel flow is a great factor for preventing the occurrenceof breakout and internal defect of a cast slab, and also it is importantto stably flow a mold powder thereinto. For this end, the continuouscasting is particularly necessary to be carried out under the abovecondition, whereby the disorder of oscillation mark is removed and themark depth is reduced. This condition is preferable to be S·F≧1000.

Moreover, as the value of oscillation number (vibration frequency) Fbecomes higher, the consumption of powder becomes large and the depth ofoscillation mark is reduced, so that it is preferably not less than 150cpm, more particularly not less than 200 cpm. And also, the maximumvalue is about 600 cpm from viewpoints of mitigation of disorder degreeof oscillation waveform and maintenance of powder consumption and thelike.

When the high-speed casting is particularly carried out at a throughputof molten steel of not less than 6 t/min, preferably not less than 7t/min, more particularly not less than10 t/min for the production ofsurface-carefree cast slab assuming the direct rolling, the above effectbecomes more remarkable, and also it can be prevented to deeply invademolten steel of higher temperature into a position lower than adischarge side of the mold for continuous casting, whereby the remeltingof solidification shell is avoided. Moreover, the throughput of moltensteel of 6 t/min is a case assuming the continuous casting for slabshaving a thickness of 0.22 m and a width of 1.2 m, in which the castingrate V_(c) is about 2.9 m/min.

In FIGS. 6a and b is shown a construction of an installation (mold forcontinuous casting) suitable for carrying out the invention.

In this figure, numeral 1 is a mold for continuous casting combining apair of short-side walls 1a and a pair of long-side walls 1b, numeral 2an immersion nozzle feeding molten metal into the mold 1 for continuouscasting, numeral 3 an electromagnet (superconducting electromagnet)applying static field between mutual long-side walls 1b of the mold 1for continuous casting, in which the electromagnet 3 is disposed at therear of the mold 1 for continuous casting.

In the installation shown in FIGS. 6a and b, when static field having amagnetic flux density of not less than 0.5 T is applied with theelectromagnet 3 (meniscus portion: 0.5 T, lower portion of molten steeljetted: 0.5 T) during the feeding of molten steel through the immersionnozzle 2, braking force is applied to the molten steel jet by anelectromagnetic force (Lorenz force) resulting from an induction currentgenerated by the interaction between the static field and the moltensteel jet to form a decelerated uniform flow and hence there is causedno entrapment of the mold powder and the deep invasion of inclusion forcatching with the solidification shell.

FIGS. 7a and b are a case that the static field is applied to a fullregion in widthwise direction of the long-side wall 1b in the mold forcontinuous casting (provided that static field of not less than 0.5 T isapplied to the meniscus portion and the lower portion of molten steeljetted). In this case, the flow of molten steel jetted out from theimmersion nozzle 2 is rectified while flowing in uniform magnetic fieldirrespectively of the variation of operation conditions such asdischarge angle, discharge rate and the like.

When the electromagnets 3 are arranged on upper and lower positions fromthe discharge port 2a of the immersion nozzle 2 as shown in FIGS. 8a andb, the jet of molten steel can be enclosed between the upper and lowerelectromagnets, so that the reduction of invasion depth of the jetcontaining inclusions and the tranquilization of meniscus aresimultaneously attained but also the temperature drop of molten steel inthe mold can be controlled.

Although the multihole type immersion nozzle is shown in all of FIGS.6-8, the single-hole type immersion nozzle can be used in the invention,and the similar results are obtained.

In FIGS. 9a and b is shown a case of using a single-hole type straightnozzle as the immersion nozzle.

In such an immersion nozzle, the jet of molten steel invades into adeeper position, so that there is a fear of remelting the solidificationshell and invading inclusions and bubbles, but the flow rate of moltensteel is decelerated by the static field located beneath the immersionnozzle and, at the same time the invasion of inclusions and gas bubblesis prevented and the downstream flow is uniformized. On the other hand,the reflection current (induction current) and the upstream flow formedby the magnetic field are weakened by the static field in the meniscusportion and hence the disorder of molten steel surface becomes small.

When the electromagnets are arranged at up-and-down positions as shownin FIGS. 9a and b, the arrangement may be a region more effectivelydeveloping the application of magnetic field from the arranging relationto the immersion nozzle, but it is desirable that the magnetic poles aredifferent in the up-and-down positions and the opposed faces,respectively.

FIG. 10 shows a construction of the electromagnet 3 for the generationof static field suitable for carrying out the invention. The magnet 3comprises a helium tank, a radiant heat shield and a vacuum containersurrounding them to prevent the entering of heat due to convection, inwhich the helium tank is connected to a liquid helium container and theradiant heat shield is connected to a liquid nitrogen container,respectively. The magnet 3 is always cooled by the liquid helium to beheld at not higher than -268.9° C. A liquid nitrogen is always fed fromthe liquid nitrogen container to the radiant heat shield so as not todirectly provide heat from exterior to the helium tank. Each of thecontainers is provided with a refrigerating machine (not shown), wherebyeach vaporized gas is again cooled and liquefied for recover into eachcontainer.

When the superconducting electromagnet as shown in FIG. 10 is used as anelectromagnet for the generation of a static field, a higher magneticflux density is obtained, but also an iron core is not used, so that theweight reduction can be attained as compared with the conventionalnormal-conducting type electromagnet. Further, it is not necessary toalways pass current, so that energy-saving is very advantageouslyattained.

In the application of a static field, it is advantageous to use theabove superconducting electromagnet. The normal-conducting electromagnetcomprises an iron core, a coil surrounding the iron core, a power sourcepassing current to the coil and the like. In such a normal-conductingelectromagnet, it is necessary to increase the winding number of coilsor increase the size of the iron core or increase the current valuepassing to the coil in order to provide a larger braking force. Thereare the following problems in the continuous casting using thenormal-conducting electromagnet:

1) Since the normal-conducting electromagnet is directly attached to theback of the mold for continuous casting, Lorenz force moving moltensteel in the mold in up and down directions is generated with theup-and-down vibration (oscillation) of the mold to promote the variationof molten steel surface. Further entrapment of the mold powder becomesprevalent.

2) Since the iron core of the normal-conducting electromagnet has aweight of not less than several dozens of tons, inertia forceaccompanying the vibration of the mold increases, so that there is alimit for increasing the vibration frequency of the mold.

3) When the high-speed casting is carried out at a throughput of moltensteel exceeding 6 ton/min, it is necessary to apply a static field so asto have a magnetic flux density of not less than 0.5 T, so that itshould be attempted to increase the number of winding coils or the sizeof the iron core and hence the problems of the above items 1) and 2)become more conspicuous. In addition, a large force is applied to thecooling plate constituting the mold to bring about the deformationthereof (stress acting on the cooling plate becomes large in proportionto square of an intensity of magnetic field), during which molten steelis leaked out from a gap formed in the mold to break the solidificationshell and bring about the breakout.

In the invention, the superconducting electromagnet is used in order tosolve the aforementioned problems. In this case, the superconductingelectromagnet is arranged independently of a support system for the moldand a mutual distance between the superelectromagnets may be changed byreciprocally approaching and separating them in accordance with thecasting condition to adjust the magnetic flux density of the staticfield.

When the superconducting electromagnet is used as a means for applyingthe magnetic field to the mold for continuous casting, it is possible toattain the compactness of the installation (total weight can becontrolled to not more than several tons) and the braking force tomolten steel can largely be improved, so that the deterioration ofquality due to the entrapment of inclusion or the like is mitigated andit can easily be coped with the high-throughput, high-speed casting.

The superconducting electromagnet is arranged on each rear surface ofthe opposed side walls in the mold for continuous casting. However, whenthe superconducting electromagnet oscillates accompanied with theoscillation of the mold, the superconducting state is broken to causeso-called quenching, so that the support system for the mold (not shown)is separated from the support system for the superconductingelectromagnet as shown in FIG. 11, whereby the mutual superconductingelectromagnets can reciprocally be approached to or separated away fromeach other.

As shown in FIG. 11, the superconducting electromagnet 3 is placed on atruck 4 disposed on the rear of the mold 1 for continuous casting, andthe truck 4 is reciprocally moved along a rail 5 to change a distancebetween magnetic poles, if necessary, whereby the magnetic flux densitycan simply be adjusted even in the casting. In FIG. 12 is shown aperspective view of FIG. 11.

Furthermore, the superconducting electromagnet 3 is not affected by theoscillation of the mold 1 owing to the adoption of the aboveconstruction, so that Lorenz force moving molten steel in up and downdirections in the mold is not generated and the force deforming thecooling plate of the mold is not applied and hence the continuouscasting can stably be conducted.

A great merit of using the movable superconducting electromagnet is asfollows:

After current slows in the superconducting electromagnet, when thesupply of the current is stopped to render the magnet into anelectrically shortcircuit and insulating state, magnetic field cansemi-permanently be applied without continuously flowing the current.However, when the arranging position of the superconductingelectromagnet is constant (fixed), if its is necessary to adjust themagnetic flux density in accordance with the casting condition (exchangeof tundish or immersion nozzle during the continuous casting, or a casethat an operator should approach to the mold), the insulating state mustbe released to change current value. In this case, electric energy isexcessively consumed, so that there is caused an inconvenience ofdamaging the merit in the use of the superconducting electromagnet.According to the invention, the superconductive electromagnets canreciprocally be approached to or separated away from each other, so thatthe magnetic flux density can simply be adjusted without wastefullyconsuming wasteful energy.

In the mold for continuous casting shown in FIG. 11, the state ofvarying the magnetic flux density (relative magnetic flux density) whenchanging the distance between magnetic poles of the superconductiveelectromagnets is shown in FIG. 13, and the state of deforming thecooling plate of the mold when the superconducting electromagnet isfixed to the mold for continuous casting (the support system for thesuperconducting electromagnet is the same as the support system for themold) is shown in FIG. 14, respectively.

Next, there will be described a case that the flow of molten steeljetted out from the discharge port of the immersion nozzle is controlledby applying current in the mold for continuous casting in thehigh-throughput, high-speed casting accompanied with the application ofa static field.

In FIGS. 15a and b is shown a state of arranging electrodes 6 for theapplication of current in the mold 1 for continuous casting. Theelectrode 6 is comprised of a conducting portion 6a and an insulatingportion 6b as shown in FIG. 16. In case of using the multihole typeimmersion nozzle, the conducting portions 6a of the electrode 6 arearranged on the upward and downward positions of the discharge port 2a.

When the mold for continuous casting is used so as to have aconstruction as shown in FIGS. 15a and b and current is applied (currentflows from the conducting portion 6a at the upper position of thedischarge port 2a toward the conducting portion 6a at the lowerposition, i.e. current flows in a direction along a drawing direction ofa cast slab) with the application of a static field having a magneticflux density of not less than 0.5 T to the meniscus portion and thelower portion of molten steel jetted, even if the high-speed casting iscarried out at a throughput of molten steel exceeding 6 t/min, the flowrate of molten steel jetted out from the discharge port of the immersionnozzle becomes very small and hence inclusions and the like included inmolten steel are not caught with the solidification steel without deeplyinvading thereinto.

In FIGS. 17a and b is shown a case of using a single-hole type immersionnozzle 2. In the continuous casting using the mold of such aconstruction, current i slows in a direction perpendicular to thelong-side wall 1b of the mold 1, whereby the flow rate of molten steeljetted is reduced like in the case shown in FIGS. 15a and b.

In the continuous casting using the single-hole type immersion nozzle 2,FIGS. 18a and b show a case where a static field is applied to themeniscus portion in the mold 1 and the full width at the lower endthereof, while current is applied between mutual opposed walls of thesolidification shell S just beneath the delivery side of the mold 1through electrode rolls 7a, b.

In continuous casting using a mold of such construction, there areadvantages in that the flow of molten steel jetted out from theimmersion nozzle 2 is offset by an upstream flow generated by theapplication of static field and the flow of current and also thestirring of molten steel in the mold can be expected, while a uniformdownstream flow can be obtained without causing the variation of moltensteel surface due to the upstream flow.

In FIGS. 19a and b is shown in case that static field is applied to fullwidth of an upper part (meniscus portion) of the mold 1 and a regionincluding the discharge port of the immersion nozzle 2, while current iflows in a direction perpendicular to the long-side wall 1b of the mold1 in the continuous casting using the single-hole type immersion nozzle2. In such a continuous casting, it is possible to attain not only thereduction of the flow rate of molten steel jetted but also the controland tranquilization of variation of molten steel surface in the mold.

Moreover, the region of applying static field and the region of flowingcurrent differ in accordance with the construction of the immersionnozzle and the casting condition, so that they are not limited to onlythe cases of FIG. 15 to FIG. 19.

FIG. 20 is a graph showing a relation between a magnetic flux density ofstatic field and a value of current when molten metal of a low meltingpoint alloy having substantially the same properties as molten steel issubjected to continuous casting (when castable flow rate at the lowerend of the mold is previously determined by conducting fluid and heattransfer calculations based on data obtained in actual machine, the flowrate lower than the determined value is castable) using the single-holetype immersion nozzle (casting model experiment).

When current flows in the mold for continuous casting, it is consideredthat a value of current not withstanding to the operation due toself-heat buildup of the electrode or cable is restricted to about 2000A even from a viewpoint of heat transfer of molten steel. In theinvention, even when the current value is range of above limit range of2000 A, the flow of molten steel jetted can be controlled by applyingstatic field so as to have a magnetic flux density of not less than 0.5T, so that it can easily be coped with the high-speed casting at athroughput of molten steel of 6-10 ton/min.

In the invention, it is preferable that current applied to the mold isabout 400 A-2000 A from viewpoints of the above self-heat buildup ofcable, electrode or the like and an efficiency of upstream flowgenerated by static field and current and so on.

In FIG. 21 is shown results examined on the state of generating coildefect ratio when extremely-low carbon steel is subjected to continuouscasting by varying the magnetic flux density of static field applied inthe mold for continuous casting (meniscus portion: 0.5 T, lower portionof molten steel jetted: 0-10 T, FIG. 6) and the resulting cast slab isfinished to a cold rolled coil.

The occurrence ratio of coil defect is considerably decreased on aborder at the magnetic flux density of about 0.5 T (both the meniscusportion and the lower portion of molten steel). Particularly, whencurrent flows in the mold, the deflected flow of molten steel issuppressed and the coil defect ratio is further reduced.

Then, there is described a case where flow of molten steel iseffectively controlled by arranging electrical terminals on theshort-side walls of the mold for continuous casting so as to form aclosed circuit flowing induction current thereinto during theapplication of static field.

At first, when two-hole type immersion nozzle is used as the immersionnozzle 2 as shown in FIG. 22, the discharge port 2a faces to theshort-side wall 1a of the mold, so that molten steel jetted out from theimmersion nozzle 2 into the mold also faces to the short-side wall 1a ofthe mold to divide into upstream flow and downstream flow as shown byarrows.

As to the downstream flow, there is a problem that inclusions or bubblesincluded in molten steel are deeply invaded in craters to cause internaldefects in the resulting cast slab. Therefore, the downstream flow canbe decreased by Lorenz force generated by an interaction between astatic field and the molten steel jet when the static field is appliedto molten steel in the mold by the electromagnet 3. In the high-speedcasting under conditions that the throughput of molten steel is 6 t/minand the static field is applied so as to have a magnetic flux density ofnot less than 0.5 T, however, there arise the following problems.

When static field B is applied for decreasing the flow rate v of thedownstream flow as shown by a perspective view in FIG. 23a, inductioncurrent I flows by an interaction between downstream flow rate v andstatic field B and hence a force F is created in a direction opposite tothe flowing direction of molten steel by an interaction between theinduction current I and the static field B to decrease the downstreamflow rate. However, the induction current I forms an electric circuit inmolten steel to generate currents I₁, I₂, I₃, I₄ in a direction oppositeto the induction current I as shown by longitudinal section in FIG. 23band by transverse section in FIG. 23c.

Since magnetic flux passes from the electromagnet through regions ofcurrents in a direction opposite to the induction current I or so-calledreflection currents, a force directing opposite to a braking force forthe flow of molten steel is created by an interaction between thereflection current and the static field. This means that the brakingforce for the molten steel flow is offset by the presence of thereflection current. The intensity of the reflection current becomeslarge as the downstream flow becomes fast and the magnetic field appliedbecomes strong, so that even if it is intended to more effectivelycontrol the molten steel flow, the reflection current may be an obstacleto providing good results.

In the invention, therefore, electrical terminals leading the inductioncurrent are arranged on the short-side walls of the mold andcommunicated with each other through a conducting means to flow theinduction current in molten steel from one of the terminals to the otherterminal.

A preferable case is shown in FIG. 24 as a partial section view.

In this apparatus, the lower electromagnet 3 applies a braking force tothe downstream flow of molten steel likewise the case of FIG. 22, whilerolls 8 are arranged just beneath the short-side walls 1a of the moldsituating the electromagnets 3 and pressed to a cast slab and connectedto each other through a conductor 9.

The rolls 8 of FIG. 24 are pressed to the cast slab and rotated inaccordance with the drawing of the cast slab, so that the supply of theinduction current is not interrupted.

Another example of the electrical terminal is shown in FIG. 25. Theterminal of FIG. 25 is constructed so as to successively press aplurality of plates 10 in accordance with the drawing of the cast slab,in which each of the plates is connected to a connector 11 so as not tointerrupt the supply of the induction current. An endless track mayconcretely be mentioned.

A means for actuating the plural plates is optional. When the terminalis a plate as shown in FIG. 25, a large contact area is advantageous.

According to such a construction, the induction current is not caused inmolten steel inside the mold but forms a circuit passing through theterminal and conductor as shown in FIG. 26, so that the reflectioncurrent generated in molten steel inside the mold is not created andhence the electromagnetic force is not caused in the same direction asthe molten steel flow and the braking force for the molten steel flow isnot offset, and consequently the control of the molten steel flow caneffectively be conducted.

In the invention, the arranging position of the electrical terminal isnot particularly restricted as long as the terminal is located on theshort-side wall of the mold and in the vicinity of a region generatingthe induction current.

The apparatus is not limited to the illustrated embodiment and may takevarious modifications. For instance, the immersion nozzle may be aso-called straight nozzle having a single discharge port in addition tothe nozzle having two discharge ports.

Next, the invention will be described in terms of a concrete apparatuswhen the high-throughput, high-speed casting is carried out by applyingoscillations of not less than 150 cpm to the mold for continuouscasting.

As previously mentioned, it is an effective means to enhance theoscillation number (vibration frequency) of the mold for continuouscasting in order to ensure the stability of the operation and obtain acarefree cast slab having good surface properties in the high-speedcasting.

In order to stabilize the growth of shell at an initial solidificationand prevent the restraint breakout, it is desirable that a negativestrip ratio (NS value) represented by the following equation is at leasta positive value, preferably a higher value. The need of rendering thenegative strip ratio into the positive value means that the descendingrate of the mold is necessary to ensure a time faster than the castingrate.

    NS={(2·S·f/v)-1}×100

where

S:up-and-down stroke of the mold for continuous casting (mm)

F: oscillation number (cpm)

v: casting rate (cm/s)

As seen from the above equation, when the casting rate v is simplyincreased, the negative strip ratio lowers, so that it is necessary toenhance either the mold oscillation stroke S or the oscillation number For both.

However, when the stroke S of the mold is made large, there is a fear ofbringing about the biting of solid powder in the meniscus portion ofmolten steel inside the mold or the clogging of powder channel due toslug rim, so that the stroke S of the mold should be made as small aspossible. It is usually set to not more than 10 mm. As a result, it isrequired to enhance the oscillation number (vibration frequency) F ofthe mold for continuous casting in order to conduct the casting aimed atthe invention. Further, it is advantageous to enhance the oscillationnumber F of the mold even in the decrease of oscillation mark depth.

In short, it is necessary to simultaneously satisfy the secure of thestability in the casting and the improvement of surface properties inthe cast slab in order to realize the high-speed continuous casting byincreasing the throughput amount per 1 strand. For this purpose, it isimportant to enhance the oscillation number of the mold.

To this end, a so-called air-core superconducting electromagnet havingno iron core is utilized in the invention.

In FIG. 27 is sectionally shown an example of a main part of thecontinuous casting apparatus according to the invention.

In the illustrated apparatus, the electromagnet 3 has no iron core andis comprised of only a coil 3a formed by superconducting wire. As a mainpart of the electromagnet 3 is shown in FIGS. 28a and b, the windingnumber is greater as compared with the wound coil of the conventionalelectromagnet (multi-winding) and a given magnetic flux densitycorresponding to the high-throughput, high-speed casting is obtained.

When using such an air-core type electromagnet, the weight of theelectromagnet is decreased to 1/5-1/7 of a conventional electromagnetand the total weight of mold and electromagnet in the oscillation of themold is mitigated by the decreased weight of the electromagnet, wherebythe oscillation number of the mold can be enhanced.

In case of slabs having a size of 200-300 mm t×700-1800 mm w, theoscillation number in the conventional continuous casting apparatus isabout 130-150 cpm at maximum, while the air-core electromagnet canensure the oscillation number of not less than 200 cpm, particularlymore than 220-230 cpm.

FIG. 29 shows an example provided with an electromagnet 3 comprised of asuperconducting coil 3a by planely winding a superconducting wire asshown in FIG. 30.

In the superconducting coil 3a, a superconducting material such as Nb,Ti or the like may be used as a wire filament. The superconducting stateis maintained by arranging a cooling box on the rear of the coil to coolwith a liquid helium or the like. Moreover, the concrete construction ofthe cooling mechanism and the like in FIG. 29 is substantially the sameas in FIG. 10.

When the apparatus provided with the superconducting electromagnet iscompared with the apparatus provided with the electromagnet having aniron core, the weight can be reduced to about 90%, so that a big weightreduction can be attained but also the magnetic flux density can be madehigher by 3-5 times than the conventional one (not more than about 0.3T).

The arrangement of the air-core superconducting electromagnet to themold may take various modification in addition to the illustratedembodiments.

BEST MODE FOR CARRYING OUT THE INVENTION EXAMPLE 1

A slab having a thickness of 220 mm and a width of 1600 mm is cast in anamount of 260 tons per one charge by using molten steel having achemical composition of C: 10-15 ppm, Mn: 0.15-0.2 wt %, P: 0.02-0.025wt %, S: 0.008-0.012 wt %, Al: 0.025-0.035 wt % and T.O:25-31 ppm andconducting 600 charges of continuous casting in a continuous castingmachine provided with a mold having a construction as shown in FIG.6-FIG. 9, in which a distance between long-side walls (corresponding toa thickness of a cast slab) is 220 mm, a distance. between short-sidewalls (corresponding to a width of the cast slab) is 1600 mm and asuperconducting electromagnet for the generation of static field havinga length of 200 mm and a width of 2000 mm (kind of coil: Nb--Ti wire) isarranged on the rear of the long-side wall, under the followingconditions:

Magnetic flux density: 0.5 T in meniscus portion, 1.0 T in lower portionof molten steel jetted

Throughput of molten steel: 8 t/min

Two-hole type immersion nozzle (FIG. 6-FIG. 8)

Single-hole type immersion nozzle (FIG. 9)

Nozzle size: 80 mm in inner diameter

Size of discharge port in immersion nozzle: square having a side of 80mm (two-hole type immersion nozzle)

Discharge angle of immersion nozzle: 20° downward (two-hole typeimmersion nozzle)

Position of discharge port in immersion nozzle: 230 mm from meniscus upto an upper end of discharge port of nozzle

Position of meniscus: position of +20 mm from upper end of coil

Oscillation number of mold: 220 cpm

Stroke of mold: 7 mm

Casting rate: 2.89 m/min and the nozzle clogging in the casting, thestate of generating breakout and internal and surface qualities (coildefect ratio) of the resulting slab are examined. The results areshowing in Table 1 with qualities of slab obtained by a comparativemethod conducting the continuous casting under the same conditions asmentioned above except that stating field is not applied.

                  TABLE 1                                                         ______________________________________                                        Items                                                                                                     Occur-                                                                              Occur-                                      Applica-         Index of   rence rence                                       tion             molten steel                                                                             ratio of                                                                            ratio of                                    type of Index of temperature                                                                              coil  break-                                                                              Gas                                   magnetic                                                                              nozzle   in mold    defect                                                                              out   blow-                                 field   clogging*                                                                              (°C.)                                                                             (%)   (%)   ing**                                 ______________________________________                                        FIG. 6  0.03     1 ± 1   0.01  ≦0.03                                                                        blowing                               FIG. 7  0.03     1 ± 1   0.01  ≦0.03                                FIG. 8  0.03     1 ± 1   0.007 ≦0.03                                FIG. 9  0        1 ± 1   0.01  ≦0.03                                no appli-                                                                             0.42     10 ± 3  0.16  0.3                                         cation of                                                                     static field                                                                  (multihole                                                                    nozzle)                                                                       FIG. 6  0.04     2 ± 1   0.02  ≦0.03                                                                        no                                    FIG. 7  0.04     2 ± 1   0.02  ≦0.03                                                                        blowing                               FIG. 8  0.04     1 ± 1   0.02  ≦0.03                                FIG. 9  0        1 ± 1   0.08  ≦0.03                                no appli-                                                                             0.45     12 ± 3  0.15  0.4                                         cation of                                                                     static field                                                                  (multihole                                                                    nozzle)                                                                       ______________________________________                                         *6 continuous casting                                                         **gas flow rate 24 Nl/min                                                

As seen from Table 1, according to the invention, the nail depth ofoscillation is made shallow and the entrapment of powder and thevariation of molten steel surface can be reduced, so that it is possibleto improve the surface quality and also the internal quality can be madehigher. As a result, it has been confirmed that carefree cast slab canstably be produced in the high-throughput, high-speed continuouscasting.

EXAMPLE 2

A slab having a thickness of 220 mm and a width of 800-1800 mm isproduced by casting an extremely-low carbon Al killed steel (C: 0.001 wt%) in an installation provided with a mold for continuous casting shownin FIG. 11 under conditions that a magnetic flux density of static fieldis 0.2-1.0 T (distance between mutual superconducting electromagnets isadjusted at up and down positions), a throughput of molten steel is 3.0t/min-8.0 t/min, an oscillation number is 150-240 cpm and a stroke is7-9 mm, which is then finished into a steel sheet through rolling stepand annealing step (continuous annealing line), and thereafter thesurface quality of the steel sheet (occurrence ratio of surface defectin steel sheet) is examined.

The results are shown in FIG. 31 together with results when thecontinuous casting is carried out by using a normal-conductingelectromagnet and fixing to the mold for continuous casting to applystatic field having a magnetic flux density up to about 0.4 T (limit inthe prior art).

As seen from FIG. 31, it has been confirmed that in the continuouscasting according to the invention, the occurrence ratio of defect islow within a range of 0.2-0.4 T as compared with the case of conductingthe continuous casting by the application of static field through thenormal-conducting electromagnet and that when the magnetic flux densityis increased to 1.0 T, it is possible to effectively decelerate the flowof molten steel jetted out from the immersion nozzle and hence theentrapment of inclusions and the like can be mitigated to more reducethe occurrence ratio of defect.

EXAMPLE 3

A continuous casting is carried out by using an apparatus having aconstruction as shown in FIG. 24 according to methods A-E under thefollowing conditions.

Conditions

Kind of steel to be cast:

Extremely-low carbon aluminum killed steel (C: 15-25 ppm, P: 0.015-0.020wt %, S: 0.01-0.015 wt %, Al: 0.03-0.04 wt %, T.O: 25-28 ppm)

Continuous casting machine:

vertical bending type continuous casting machine having a verticalportion of 2.5 m

Size of mold:

width of 1600 mm and thickness of 220 mm corresponding to size of castslab

Immersion nozzle: 25° downward, two-hole nozzle

Casting rate: 3.5 m/min

Oscillation number of mold: 220 cpm

Stroke of mold: 8 mm

Application of static field:

static field is applied so that the magnetic flux density is equal inboth the meniscus portion and the lower portion of molten steel jetted.

Throughput: 8.62 t/min

Method A: no electromagnet

Method B: normal-conducting electromagnet, magnetic flux density: 0.3 T

Method C: normal-conducting electromagnet, magnetic flux density: 0.3 T,current is flowed by pressing a plate terminal to a cast slab

Method D: superconducting electromagnet, magnetic flux density: 1.1 T

Method E: superconducting electromagnet, magnetic flux density: 1.1 T,current is flowed by pressing a plate terminal to a cast slab

The cast slab obtained by each of the above methods is cut into a sliceat a pitch of 10 mm in thickness direction, from which is measured thenumber of inclusions in the slab by an X-ray permeation process. Themaximum value measured is shown in FIG. 32 by an index on the basis thatthe value of the method A is 1. From this figure, it is understood thatthe internal quality of the cast slab in the methods D, E isconsiderably improved as compared with those in the methods A-C.

Furthermore, after the cast slab obtained by each of the methods issubjected to hot rolling and cold rolling, magnetic flaw detecting test(MT test) is made, from which it has been confirmed that there is atendency similar to FIG. 32.

EXAMPLE 4

A continuous casting of 7200 charges (260 tons per one charge) iscarried out by casting molten steel having a chemical composition of C:10-15 ppm, Si: 0.008-0.005 wt %, Mn: 0.15-0.2 wt %, P: 0.02-0.025 wt %,S: 0.008-0.012 wt %, Al: 0.025-0.035 wt % and T: 25-31 ppm in acontinuous casting machine provided with a mold having a constructionshown in FIG. 15, FIG. 17, FIG. 18 and FIG. 19, in which a distancebetween long-side walls (a thickness of a cast slab) is 220 mm, adistance between short-side walls (a width of the cast slab) is 1600 mmand a superconducting electromagnet for the generation of static fieldhaving a length of 200 mm and a width of 2000 mm (Nb--Ti wire) isarranged on the rear of the long-side wall, under the followingconditions, and the nozzle clogging of immersion nozzle in the casting,the state of generating breakout and internal and surface qualities(coil defect ratio) of the resulting slab are examined. The results areshown in Table 2 with results of a comparative example conducting thecontinuous casting under the same conditions as mentioned above exceptthat stating field is not applied.

Conditions

Magnetic flux density: 1.0 T (equal static field is applied in bothmeniscus portion and lower portion of molten steel)

Throughput of molten steel: 8 ton/min

Value of current applied at electrode: 800 A

a. Two-hole type immersion nozzle

nozzle size: 80 mm in inner diameter

size of discharge port in immersion nozzle: square having a side of 80mm

discharge angle of immersion nozzle: 20° downward

position of discharge port in immersion nozzle: 230 mm from meniscus upto an upper end of discharge port of nozzle

position of meniscus: position of +20 mm from upper end of coil applyingstatic field

b. Single-hole type immersion nozzle

nozzle size: 80 mm in inner diameter

position of discharge port in immersion nozzle: 230 mm from meniscus upto a top end of nozzle

position of meniscus: position of +20 mm from upper end of coil applyingstatic field

                  TABLE 2                                                         ______________________________________                                               Items                                                                                    Index of                                                                      molten steel                                                                             Occurrence                                                                            Occurrence                               Application                                                                            Index of temperature                                                                              ratio of                                                                              ratio of                                 type of  nozzle   in mold    coil defect                                                                           breakout                                 magnetic field                                                                         clogging*                                                                              (°C.)*.sup.1                                                                      (%)*.sup.2                                                                            (%)*.sup.3                               ______________________________________                                        Acceptable                                                                             0.03     2 ± 1   0.01    ≦0.03                             Example                                                                       FIG. 15                                                                       Acceptable                                                                             0.03     2 ± 1   0.01    ≦0.03                             Example                                                                       FIG. 17                                                                       Acceptable                                                                             0.03     2 ± 1   0.007   ≦0.03                             Example                                                                       FIG. 18                                                                       Acceptable                                                                             0.03     2 ± 1   0.01    ≦0.03                             Example                                                                       FIG. 19                                                                       Comparative                                                                            0.42     10 ± 1  0.16    0.3                                      Example                                                                       ______________________________________                                         *6 continuous casting                                                          Index of nozzle clogging: (S.sub.b -S.sub.a)/S.sub.b                          S.sub.b : Area of discharge port in nozzle before casting                     S.sub.a : Area of discharge port in nozzle after casting                     *.sup.1 Index of molten steel temperature in mold: T.sub.t -T.sub.m           (°C.)                                                                   .sup. T.sub.t : tundish temperature                                           .sup. T.sub.m : temperature in mold                                          *.sup.2 Occurrence ratio of coil defect: D.sub.p /N × 100                .sup. (cold rolled coil rolled to sheet is called as coil simply)             .sup. N: total coil                                                           .sup. D.sub.p : defect occurrence ratio                                      *.sup.3 Occurrence ratio of breakout: N.sub.b /N × 100(%)                .sup. N: total number of casting charges                                      .sup. D.sub.p : casting charge generating breakout                      

As seen from Table 2, in the continuous casting according to theinvention, the entrapment of mold powder and the variation of moltensteel surface can be reduced even in the casting having a throughput ofmolten steel of 8 ton/min, so that good internal and surface qualitiescan be ensured. It has been confirmed that carefree cast slab can stablybe produced in the high-speed continuous casting.

EXAMPLE 5

A continuous casting is carried out by methods A-C under the followingconditions.

Conditions

Kind of steel to be cast:

Extremely-low carbon aluminum killed steel (C: 20-25 ppm, P: 0.02-0.03wt %, S: 0.008-0.010 wt %, Al: 0.025-0.035 wt %, T.O: 30-40 ppm)

Size of mold:

width of 1500 mm and thickness of 200 mm corresponding to size of castslab

Weight of mold (excluding an electromagnet):

11 t per one mold

Casting rate: 3.6 m/min

Throughput: 7.56 t/min/strand

Stroke of mold: 9 mm

Oscillation number of mold: 230 cpm

Arrangement of electromagnet:

full width of long-side wall of mold, 2 up-and-down stages (FIG. 27,FIG. 29)

Magnetic flux density:

0.4 T (limit value) to meniscus portion and lower portion of moltensteel in normal-conducting electromagnet, 0.7 T to both the meniscusportion and the lower portion of molten steel jetted in superconductingelectromagnet

Method A: normal-conducting electromagnet having an iron core, weight ofthe magnets (total weight) is 19 t on both long-side walls of the mold

Method B: normal-conducting electromagnet having no iron core, weight ofthe magnets (total weight) is 3 t on both long-side walls of the mold

Method C: superconducting electromagnet, air-core, weight of the magnets(total weight) is 2 t on both long-side walls of the mold

In these methods A-C, total weight of mold and electromagnet, upperlimit of vibration frequency, upper limit of negative strip ratio andmaximum magnetic flux density in the mold are measured. The results areshown in Table 3.

                  TABLE 3                                                         ______________________________________                                                        Maxi-     Maxi-  Maxi-                                               Total    mum       mum    mum                                                 weight of                                                                              value of  value of                                                                             value of                                            mold and oscillation                                                                             negative                                                                             magnetic                                            electro- number    strip  flux                                                magnet   for mold  ratio  density                                      Kind   (t)      (cpm)     (%)    (T)    Remarks                               ______________________________________                                        Method 30       150       -25    0.30   compara-                              A               (2.5 Hz)                tive                                                                          Example                               Method 14       220       10     0.14   Accept-                               B               (3.7 Hz)                able                                                                          Example                               Method 13       230       15     1.1    Accept-                               C               (3.8 Hz)                able                                                                          Example                               ______________________________________                                    

The occurrence ratio of breakout in each of these methods is shown inFIG. 33, and the results examined on the surface properties of the castslab are shown in FIG. 34, respectively. Moreover, the occurrence ratioof breakout (ratio of casting heat) is represented by a relativeevaluation as a standard of 0.9% in the method A, while the surfaceproperties of the cast slab are represented by a relative evaluation onthe basis that the value of the method A is standard when the number ofinclusions and bubbles adhered to the surface of the cast slab after thehot scarfing of the slab is measured to determine the adhesion numberper unit area. From Table 2 and FIG. 33 and FIG. 34, it is understoodthat in the methods B, C according to the invention, the weight of theelectromagnet can be reduced and the oscillation of the mold can be madehigher, whereby the negative strip ratio can be set to a higher cycleand hence the occurrence ratio of breakout is considerably decreased ascompared with that in the method A.

As to the surface properties of the cast slab, the effect of reducingthe oscillation mark depth by the high cycle of vibration frequency inthe mold is offset by the lowering of the magnetic flux density in themethod B, but the surface properties are improved as compared with themethod A. In case of the method C, the magnetic flux density is 1.1 Tand is very higher than 0.3 T in case of the method A, so that thesurface properties of the slab is considerably improved with the highcycle in the vibration frequency of the mold.

After the resulting cast slab is subjected to hot rolling and coldrolling, the surface defect is examined to obtain a result similar toFIG. 34.

INDUSTRIAL APPLICABILITY

According to the invention, the following effects can be expected.

1. The lowering of temperature in the molten steel surface inside themold is small, so that the occurrence of nozzle clogging is very less.Furthermore, entrapment of mold powder, entrapment of inclusions,surface defects due to oscillation and the like are mitigated andfurther remelting of steel can be avoided, so that cast slabs havinggood surface and internal qualities can stably be produced.

2. An air-core superconducting electromagnet is used as means for theapplication of a static field and is supported so as to change adistance between magnetic poles of the superconducting coilindependently of a support system for a mold for continuous casting, sothat the variation in a molten steel surface in a mold can be minimized.Furthermore, extra stress is not applied to a coiling plate of the mold,so that breakout due to the leakage of molten steel based on thedeformation of the cooling plate can be avoided. And also, theadjustment of magnetic flux can simply be made. Moreover, the brakingability can be enhanced without increasing the size of the apparatusitself, so that the cast slab having a high quality can be produced andit can easily be coped with the high-speed continuous casting having athroughput of molten steel of more than 6 ton/min.

3. An electrical terminal leading induction current is arranged on eachshort-side wall of the mold and one of the terminals on the short-sidewalls of the mold is connected to the other terminal through a conductormeans to form a closed circuit of induction current, so that the flow ofmolten steel can effectively be controlled without the occurrence of aforce obstructing the braking of molten steel flow.

4. The flow rate of molten steel jetted can more be decreased by flowingcurrent in the mold for continuous casting at a state of applying staticfield, so that even if the high-throughput, high-speed casting isconducted, mold powder is not entrapped and the inclusions are notdeeply entrapped, while the defects due to oscillation and the like aremitigated and further remelting of solidification shell can be avoidedand hence cast slabs having good surface and internal qualities canstably be produced.

5. Since the air-core superconducting electromagnet having no iron coreis used as means for applying static field to the mold for continuouscasting, the oscillation number of the mold can be increased, wherebythe oscillation mark depth can be reduced and it is possible to maintainthe negative strip ratio within a good rage even in the high-throughput,high-speed continuous casting and also the surface properties of theslab can be improved with the maintenance of the casting stability.

We claim:
 1. A method of continuously casting steel by controlling a jetof molten steel fed through an immersion nozzle into a mold forcontinuous casting while applying a static field between opposed sidewalls of the mold for the continuous casting, characterized in thatmolten steel is fed into the mold for the continuous casting at athroughput of not less than 6 t/min, and that an air-coresuperconducting electromagnet is used to simultaneously apply a staticfield having a magnetic flux density of greater than 0.5 T to a meniscusportion in the mold for the continuous casting and a static field havinga magnetic flux density of greater than 0.5 T to a lower portion ofmolten steel jetted out from a discharge port of the immersion nozzle.2. A continuous casting method according to claim 1, wherein the staticfield is applied to a full region in widthwise direction of the moldincluding the meniscus portion and the lower portion of molten steeljetted.
 3. A continuous casting method according to claim 1, wherein themold for the continuous casting is oscillated during the feeding ofmolten steel so as to satisfy the following equation:

    S·F≧450

where S: up and down strokes (mm) of the mold for the continuous castingF: oscillation number (cpm)) in the feeding of Z molten steel throughthe immersion nozzle.
 4. A continuous casting method according to claim1, wherein a gas is blown into the immersion nozzle so as to satisfy thefollowing condition:

    0.5Q≦f≦20+3Q

where f: gas blowing amount (Nl/min) Q: throughput of molten steel(t/min).
 5. A continuous casting method according to claim 1, whereinthe immersion nozzle is a single-hole type straight nozzle.
 6. Acontinuous casting method according to claim 1, wherein the air-coresuperconducting electromagnet applying the static field is arranged oneach rear of opposed side walls in the mold for the continuous castingindependently of a support system for the mold, and a distance betweenmagnetic poles of the superconducting electromagnet is changed so as toapproach with each other or separate away from each other in accordancewith casting conditions to adjust a magnetic flux density of the staticfield.
 7. A continuous casting method according to claim 1, whereincurrent is applied to the mold for the continuous casting.
 8. Acontinuous casting method according to claim 1, wherein an inductioncurrent generated by the application of the static field is taken outfrom a short-side wall of the mold for the continuous casting andsupplied to the other short-side wall thereof to circulate the inductioncurrent.